History of animals ~700 million years old




  • Chemical evidence of steroids in sediment that are found in modern sponges


  • Accepted fossils from Ediacaran period (pre-Cambrian, ~600mya)
    • mollusks, cnidarians and sponges


  • Animals then diversify across Paleozoic, Mesozoic and Cenozoic
    • in both aquatic and terrestrial environemnts
    • punctuated by mass extinctions

Animals are a monophyletic group


  • All share 3 traits


  • All animals, except sponges, have:
  1. Nerve cells called neurons
  2. Muscle cells


  • Animals are incredibly diverse
    • 35 major phyla
    • 1/3 are marine only
    • most phyla have marine members

Invertebrates: lacking a backbone





  • 95% of known animal species


  • Occupy every habitat on Earth


  • Small to huge (colossal squid)


  • Groupings characterized by tissue layers,symmetry and unique structures

Animal Symmetry


Phylum Porifera (Sponges)



  • Most ancestral animal group
    • marine
    • adults do not move, larva have flagella
    • filter feeders
    • hermaphroditic (alternate sexes)
    • lacks symmetry in body plan


  • Lots of cells, but no tissue layers
    • produce many antibiotics


  • Share similarities with protist Chonaflagellates
    • feed using cells called choanocytes

Phylum Cnidarian


  • Animals except sponges are considered “True Animals”
    • contain tissues

Phylum Cnidarian


  • Animals except sponges are considered “True Animals”
    • contain tissues


  • First lineage to diversify were Cnidarians
    • corals, hydras and jellyfish
    • pre-Cambrian origins


  • Diploblastic and radial symmetry
    • motile and sessile (non-moving)
    • medusa or polyp forms


  • Corals have symbiotic relationships with single-celled algae

Basic Cnidarian body plan ( Nematocyst on phylogeny)


polyp = non-moving form, medusa = motile form, Nematocyst = stinging cell

Most animals are triploblastic, bilaterians




  • Triploblasty = 3 germ/tissue layers


  • Complete digestive tract (most groups)
    • mouth and anus connected by tube


  • Bilateral = symmetrical in 2 halves

Most animals are triploblastic, bilaterians




  • Triploblasty = 3 germ layers


  • Complete digestive tract
    • mouth and anus


  • Bilateral = symmetrical in 2 halves


  • 3 major clades of bilaterians
  1. Lophotrochozoa
  2. Ecdysozoa
  3. Deuterostomia

Lophotrochozoa: Most share 2 structures





  • Lophophore: crown of ciliated tentacles for feeding



  • Trophophore: distinctive larval stage
    • beating cilia
    • cylinder body


  • This group is falling apart with new molecular-based phylogenies

Lophotrochozoa - 18 total phyla





  • Key phyla (18 total):
  1. Platyhelminthes (flatworms)
  2. Syndermata (rotifers)
  3. Brachiopods (lamp shells)
  4. Mollusca (snails, slugs, oysters, clams, octopuses, squids)
  5. Annelids (segmented worms)


  • Many phyla appeared in Cambrian explosion

Phylum Platyhelminthes (flatworms)


Flattened body = most cells near environment or gastrovascular cavity

Phylum Mollusca: Snails, slugs, clams, mussels, octopus & squid


Shared body plan, despite diverse forms of different species

Phylum Annelida: Earthworms ( segmentation on phylogeny)


Breath through their skin, each segment has muscles

Ecdysozoans



  • Animals with cuticle
    • shed tough external coat
    • shed as grow; molting; ecdysis


  • 8 total phyla
    • more species than all other groups


  • 2 major phyla
  1. Nematoda (roundworms)
  2. Arthropods (mostly insects)

Phylum Nematoda (round worms)





  • Cylinder bodies from 1mm to 1m


  • Aquatic, soil, plant tissues, animals tissues
    • many parasitic species (crop pests)
    • humans are host to ~50 species!


  • Longitudinal muscles; thrashing motion

C. elegans: model animal species




Share many developmental characteristics with more complex animals, including humans

Phylum Arthropoda: Rulers of the animal world


  • A billion billion arthropods estimated
    • 1 million species described
    • nearly all habitats on Earth


  • Segmented body, hard exoskeleton, jointed appendages
    • likely led to success


  • Early arthropods (Trilobites) had little variation in body plan


  • Evolution selected for functional body regions
    • head, thorax, abdomen
    • each with specialized functions

Arthropod body plans: unique Hox genes


Phylum Echinoderms: Next time


Invertebrates are not a monophyletic group!